The production of heavy oil and bitumen from a subsurface reservoir is quite challenging. One of the main reason for this is the initial viscosity of the oil reservoir is often greater than one million centipoise. Therefore, the removal of the oil from the subsurface is typically achieved by either surface mining or by the introduction of energy into the reservoir such that the reservoir is heated enough to lower the viscosity of the oil and allow it to be produced. Currently the preferred method of energy transfer to the reservoir is steam injection.
One limiting factor in the economic production of viscous oil using steam is the heterogeneous nature of the reservoir where heavy oil is found. FIG. 1 shows an example of a bitumen reservoir in Northern Alberta. As shown, there are numerous oil bearing sand layers separated by shale and mudstones. The applicability of steam injection is often limited by the impermeable shale layers and mudstones that act as barriers to vertical flow. These barriers prevent the steam from contacting sufficient amounts of heavy oil or bitumen and reducing its viscosity enough to be produced. The impermeable layers effectively compartmentalize the reservoir into thin sub-reservoirs that cannot be economically developed because of the economic requirement for significant reservoir thickness.
Vertical wells can allow the production from multiple pay zones by contacting all of the stratified layers, but they require a more efficient form of energy transfer to the reservoir in order to provide economic flow rates. As a result, current production of heavy oil and bitumen using thermal methods is limited to fairly homogeneous sand formations with good vertical permeability. Laterally continuous shale or muds can significantly reduce the amount of the resource that is considered producible. This leaves billions of barrels of oil stranded in compartmentalized reservoir sections. As an example, the McMurray formation in the Athabasca region of Alberta, Canada can have a thickness of over ninety meters. Of that, usually less than half is currently considered economically producible due to stratification of the reservoir by shale layers or mudstones.
Unlike steam, electromagnetic energies can penetrate impermeable shale and mudstone layers to heat additional hydrocarbon layers beyond. Thus, using RF radiation to target these zones, the reservoirs may be produced with vertical, slant or horizontal wells. RF has already been used in the art, although RF methods have yet to reach their full potential and are still being developed.
U.S. Pat. No. 2,757,738, for example, is a very early publication disclosing a method for heating subsurface oil reservoir bearing strata by radio frequency electromagnetic energy, where the RF electromagnetic energy is generated by a radiator within a vertical well bore. The antennas of this method are not immersed in the ore for extended distance because the well bores are vertically drilled. Additionally, the vertically drilled well bores have inherent limitations on separating the charges between horizontal earth strata.
U.S. Pat. No. 3,522,848 discloses radiation generating equipment for amplifying the oil production in a natural reservoir. In essence radio frequency electromagnetic waves are used to heat the dry exhaust gas (comprising CO2 and nitrogen) of an internal combustion engine, and the heated gas is subsequently used to heat the reservoir to reduce the viscosity of the hydrocarbons contained in the natural reservoir.
U.S. Pat. No. 4,638,863 discloses a method for stimulating the production of oil by using microwaves to heat a non-hydrocarbonaceous fluid, such as salt water, surrounding a well bore, and the heated non-hydrocarbonaceous fluid will in turn heat the hydrocarbonaceous fluid in the same formation.
U.S. Pat. No. 5,236,039 provides a system for extracting oil from a hydrocarbon bearing layer by implementing RF conductive electrodes in the hydrocarbon layer, the RF conductive electrodes having a length related to the RF signal. The spacing between each RF conductive electrode and the length of such electrodes are calculated so as to maximize the heating effect according to the frequency of the RF signal. However, the inventors' experiences indicate that standing wave patterns do not form in dissipative media, such as hydrocarbon ores, because the energy will be dissipated as heat long before significant phase shift occurs in the propagation of electromagnetic energies. Thus, this method is of limited use.
U.S. Pat. No. 7,091,460 discloses a method for heating a hydrocarbonaceous material by a radio frequency waveform applied at a predetermined frequency range, followed by measuring an effective load impedance initially dependent upon the impedance of the hydrocarbonaceous material, which is compared and matched with an output impedance of a RF signal generating unit. An important aspect of this invention relates to the fact that certain hydrocarbonaceous earth formations, for example unheated oil shale, exhibit dielectric absorption characteristics in the radio frequency range. Unlike most prior art electrical heating in situ approaches, the use of dielectric heating allegedly eliminates the reliance on electrical conductivity properties of the formations. Thus, the method supposedly allows more uniform heating and deeper penetration.
US20070289736 discloses a method of in situ heating of hydrocarbons by using a directional antenna to radiate microwave energy to reduce the viscosity of the hydrocarbon. The method preferably applies sufficient energy to create fractures in the rock in the target formation, so as to increase the permeability for hydrocarbons to flow through the rough and be produced. However, directional antennas are impractical at the frequencies required for useful penetration, because the instantaneous skin depth of penetration may be too short. For example a 2450 MHz electromagnetic energy in rich Athabasca oil sand having conductivity of 0.002 mhos/meter is 9 inches. Thus, this method is also of limited use.
WO2010107726 discloses a process for enhancing the recovery of heavy hydrocarbons from a hydrocarbon formation. Microwave generating devices are provided in horizontal wells in the formation, and a microwave energy field is created by the microwave generating devices, so that the viscosity of the hydrocarbons within the microwave energy field can be reduced and more readily produced. Electric waves must be generated for this method to work, limited its usefulness.
RE30738 describes another RF method wherein in situ heat heating of heavy oil occurs using an array of RF antenna inserted in said formations and bounding a particular volume of the oil field formations. AC currents establish electric fields in said volume, the frequency being selected as a function of the volume dimensions so as to establish substantially non-radiating electric fields, which are substantially confined in said volume. Using this method, volumetric dielectric heating of the formations will occur to effect approximately uniform heating of said volume. However, this method requires a three-row conductor design where the outer two rows are longer than the central row, so as to confine the heating between the three rows. Additionally, to achieve confinement, the spacing of conductors in the same row is less than a quarter wavelength apart, and preferably less than an eighth of a wavelength apart.
US20090242196 describes a method of producing heavy oil by determining conductivity and permittivity of the hydrocarbon-bearing rock and the overburden layer, as well as a roughness of the boundary between the hydrocarbon-bearing rock and the overburden layer. These parameters are used to construct a computer model based upon modeling the formation as a rough-walled waveguide. The model simulates propagation of radio frequency energy within the hydrocarbon-bearing rock, including simulation of radio frequency wave confinement within the hydrocarbon-bearing rock, at several frequencies and temperatures, and the retorting frequency is selected based upon the results. Radio frequency couplers are installed into at least one borehole in the hydrocarbon-bearing rock and driven with radio frequency energy to heat the hydrocarbon-bearing rock. As the rock heats, it releases carbon compounds and these are collected.
While superficially similar to the invention, reflection from rock is not preferentially used herein. One may notice in FIG. 10 below, that heating is following the rock layer. If reflection were dominant, the heating would be displaced from the rock layer, and this is not occurring. The invention is also novel relative to US20090242196 as we use horizontal directional drilling to orient the heating portion of our antennas horizontally in the horizontally planar hydrocarbon bearing strata. In addition, we use a grid pattern with adjacent planes of antennas to produce flux lines that are aligned in opposite directions to each other to prevent cancellations.
None of the abovementioned literature discloses a method or system that addresses the issue of non-producible hydrocarbons when the hydrocarbon formation is stratified with steam-impermeable shale or mudstones, especially by heating the formation with joule-heating induced by eddy currents. Thus, what is needed in the art is a method of efficiently heating heavily stratified formations.